Debugging NAS connection!

Recently I attached Sitecom MD-250 NAS unit to my wireless modem/router. I can mount my NAS in different ways (all give me access to my files):

mount // /windows/NAS –o nolinux
mount.cifs // /windows/NAS –o nolinux
mount –t cifs // /windows/NAS –o nolinux

The problem is that my system frequently hangs when I’m performing operations on NAS, leaving me with a frozen desktop. No response from the keyboard/mouse. Only two led’s (caps-lock and scroll-lock) on the keyboard are flashing.
This problem is triggered by performing one of the following operations on the NAS:

du –sh .
ls –lR DirName
rm –R DirName

However, performing these operations on a single file does not result in system freeze.
This problem can be reproduced on both Linux machines (see the signature).

Can someone help me debug this issue?

Maybe an additional option is required in the mount command?
Or, should I install some patches, and which?